1. Field of the Invention
The invention relates in general to refrigeration systems, and more specifically to refrigeration systems having electrical heaters for defrosting the evaporator coil heat exchanger assembly.
2. Description of the Prior Art
In refrigeration systems, such as transport refrigeration systems for conditioning the air of a truck or trailer hauling fresh or frozen loads, it is common to rapidly defrost the evaporator heat exchanger assembly with electrical heaters when the frost and ice build-up reduces the effectiveness of air flow over the evaporator. The evaporator section of the refrigeration system includes a plurality of refrigerant carrying tubes, often called hairpin tubes, with a large number of thin, closely spaced metallic heat exchanger plates or fins connected to the coolant tubes. The fins are usually wavy or corrugated to increase the surface area for a given heat exchanger volume. Frost builds up on the fins and impedes air flow over the surfaces of the fins, and it is this frost that must be periodically removed.
U.S. Pat. No. 3,786,227, which is assigned to the same assignee as the present application discloses periodically replacing spaced groups of fins across the heat exchanger assembly with relatively thick metallic plates, each of which has an electrical heating element attached thereto. While this is highly effective, it requires modification of the heat exchanger unit at the time of manufacture, and adds significantly to the manufacturing cost.
Another prior art approach holds an electrical heating element against the edges of the fins with a plurality of spaced metallic channel members, each of which extends across the two legs of a U-shaped electrical heating element. Each channel member includes two spring loaded fasteners, with each spring loaded fastener including a hook, a helical spring, and two large washers. While this approach does not require modification of the heat exchanger at the time of manufacture, the channel shaped device is relatively costly to manufacture and is awkward for one assembler to install. In a transport refrigeration system the evaporator heat exchanger is elevated, requiring an installer to work from below the unit. The heating element must be held against the edges of the fins which make up the lower surface of the heat exchanger, the metallic channel must be placed over the two legs of a heating element, and the two spring loaded fasteners must each be threaded up between two closely spaced wavy heat exchanger fins and attached to a coolant tube. The channel must usually be moved or adjusted along the heating element after the initial placement, until the fasteners are properly aligned with a coolant tube.
In my copending application Ser. No. 930,198 filed Nov. 19, 1986, now U.S. Pat. No. 4,716,275, which is assigned to the same assignee as the present application, I disclosed spring retainer clips which require a single hand to install, freeing the other hand of the installer for initially holding the heating element in heat exchange relation with the heat exchanger fins. Each spring retainer clip is a single piece of wire having first and second ends, with the wire having at least first, second, and third curved sections which proceed in the recited order from the first end towards the second end of the wire. The curved sections are all disposed in a common plane to enable the retainer clip to be easily inserted between two adjacent heat exchanger fins, even when the fins are of the wavy type. The first and second curved sections are respectively disposed about a coolant tube and about the heating element, with the spacing between the first and second curved sections being selected such that a spring force is developed which firmly holds the heating element against the heat exchanger fins. A curved handle is formed adjacent the second end of the wire which enables the retainer clip to be easily held as the first end is inserted between two heat exchanger fins and the first curved section is hooked over a coolant tube. The handle further enables the second curved section of the retainer clip to be pulled downwardly after the first curved section has been hooked to a coolant tube to allow the second curved section to be snapped over the heating element, to hold the heating element tightly against the heat exchanger fins.
While the spring retainer clip of my aforesaid U.S. Patent substantially reduced the cost of holding a U-shaped heating element against an assembly of finned tubes, by about 75% when compared with the hereinbefore mentioned prior art channel type clamping arrangements, it is important that a tight tolerance be held between the first and second curved sections. If a functionally equivalent spring retainer clip could be provided which did not have a critical tolerance between the two curved sections which contact the heating element and finned tube, the cost of manufacturing a spring retainer clip would be reduced by about one-half, as the spring retainer clip could be manufactured on a wire forming machine, instead of on a spring machine.
Thus, it is an object of the present invention to provide a new and improved spring clip for holding electrical heating elements in heat conducting relation with the fins of an evaporator heat exchanger assembly of a transport refrigeration system, which spring clip can be manufactured on a wire forming machine instead of requiring a spring machine.